About this replacement
JBL Synchros S400BT / Tune 600BT — 3.7V Li-Polymer Replacement Battery (P062831)
This 3.7V 600mAh (2.22Wh) Li-Polymer cell replaces the original battery in the JBL Synchros S400BT and Tune 600BT wireless headphones. Both are on-ear Bluetooth headsets that share the same compact battery form factor and power circuit. Swap this in when the original cell no longer holds enough charge for daily wireless listening.
- Synchros S400BT and Tune 600BT compatibility: Both headsets run the same 3.7V single-cell power rail with a shared connector and BMS handshake protocol. One replacement cell covers both models without modification.
- Bench tested on actual hardware: We cycled this cell through charge and discharge on the S400BT board. The BMS accepted the new pack cleanly, voltage held steady under simultaneous Bluetooth radio and audio driver draw, and no protection trips occurred.
- First-cycle charging on S400BT: Charge the headset fully via USB before the first use session. The S400BT's fuel gauge circuit calibrates its charge estimate against the cell's first full cycle — skipping this leaves the battery indicator inaccurate until a complete cycle runs.
Why the S400BT cuts out mid-track on a new battery
The S400BT draws current simultaneously from the Bluetooth radio and the audio amplifier. On a freshly installed cell that hasn't completed a full cycle, internal resistance is temporarily higher than normal. Under that combined load, the cell voltage dips briefly below the BMS cutoff threshold and the headset powers off. After two to three full charge-discharge cycles, internal resistance settles and the cutouts stop.
Battery indicator reads full but headset shuts off after a few tracks
Replacement cells ship at storage voltage — typically around 3.6–3.7V — not at a full 4.2V charge. The S400BT's gauge reads this as "charged" on first boot because it hasn't logged a baseline for the new cell. The headset then cuts off quickly because the cell genuinely has little usable charge available. Run one complete charge from flat to full via USB, and the indicator will track accurately from that point.